And that's how you get Godzilla.

The problem with that is that the subduction rifts generally also have volcanoes that spew a bunch of that material back to the surface/atmosphere. It might take a few centuries for it to go through all that, but IMO better to bury it in one place and risk future people not understanding it (they'll figure it out quickly enough if they are human or similar intelligence) than to put it somewhere where the Earth itself will eventually reject it violently and people affected won't have much choice or understanding of what happens as a result.